1. Field of the Invention
The invention relates to a photosensitive element composed mainly of an electrically conductive backing member, a photoconductive layer, and a transparent electrically insulating surface layer and, more particularly, to a photosensitive element having a charge-retentive layer in the boundary between the photoconductive layer and the transparent electrically insulating layer.
2. Description of the Prior Art
In the art of electrophotography, a photosensitive element containing a photoconductive layer made of amorphous selenium, zinc oxide, cadmium sulfide, an organic semiconductor, or the like is exposed to a light pattern to form thereon an electrostatic latent image in conformity with the configuration of the original light pattern. The latent image is then developed to form a visible image by the use of toner particles on the surface of the photoconductive layer and is transferred and fixed on a copy sheet. The photoconductive layer surface of such a photosensitive element is subjected to frequent failure and the effective life thereof limited. In order to protect the photoconductive layer from injury, an attempt to provide a transparent electrically insulating protective layer on the photoconductive layer surface has been made, but such an attempt causes an increase in residual potential after a number of copying operations have been repeated and the quality of the final copy obtained is degraded.
U.S. Pat. No. 3,041,167 teaches an electrophotographic element which effectively prevents the increase of the residual potential due to repeated copying operations, which comprises a transparent electrically insulating layer overlying the photoconductive layer so that an electrostatic latent image is produced between the electrically insulating layer and the photoconductive layer. This element is advantageous in that environmental changes do not affect the photoconductive layer, a highly sensitive photoconductive material can be used, and the thickness of the electrically insulating layer has little effect upon the performance of the electrically insulating layer. However, with this element the increase of the residual potential where the electrically insulating layer has a thickness sufficient to protect the photosensitive layer from damage can not be avoided.
An electrophotographic element, which is an improvement over the element described in U.S. Pat. No. 3,041,167 has been proposed in the art where a photosensitive plate composed of an electrically conductive backing member, a photoconductive layer overlying the backing member, a transparent electrically insulating surface layer affixed to the photoconductive layer is first uniformly electrostatically charged across the surface of the transparent electrically insulating layer and is then electrostatically charged a second time at a polarity opposite to that of the first charge or with an alternating current charge and thereafter is exposed to a light pattern to form under the transparent electrically insulating layer an electrostatic latent image in conformity with the configuration of the original light pattern.
This element is further described with reference to the drawings in which FIGS. 1 through 3 show the processes for producing an electrostatic latent image by the use of the element. In FIG. 1, a photosensitive plate composed mainly of an electrically conductive backing member 1, a photoconductive layer 2, and an electrically insulating layer 3 is first electrostatically charged by the use of a corona discharge unit 4. Preferably, the first charge is negative to negatively charge the surface of the transparent electrically insulating layer with a photoconductive layer of the P type and the first charge is positive to positively charge the surface of the transparent electrically insulating layer with a photoconductive layer of the N type. The element will be described hereinafter for simplicity in connection with a P type photoconductive layer. The condition as shown in FIG. 1 can be established by the first electrostatic charge (negative) where no sufficiently large barrier layer exists in the boundary between the backing member and the photoconductive layer and a positive charge is readily injected from the backing member side. The positive charge injected due to the first negative charge is held in the interior of the photoconductive layer in the vicinity of the boundary between the photoconductive layer and the transparent electrically insulating layer. The condition as shown in FIG. 1 can also be established by providing a uniform exposure after or at the same time of the first charge even though no positive charge is injected from the backing member side where the photoconductive layer has an ability to transport positive and negative charges or where the light is absorbed in the entire photoconductive layer. Next, in order to leave the charge only in the boundary between the photoconductive layer and the transparent electrically insulating layer, the photosensitive plate is given a second AC charge or a second charge having a polarity opposite to that of the first charge by the use of another corona discharge unit 5 to remove the charge on the surface of the transparent electrically insulating layer. As a result, the condition as shown in FIG. 2 is established. After the second charge, the photosensitive plate is exposed to a light pattern as shown in FIG. 3 to produce thereon an electrostatic latent image, which is then developed and transferred. Thereafter, the photosensitive plate is given an AC charge and at the same time is uniformly exposed.
Although the electrophotographic process is superior in stability relative to repeated operations, a large corona charging current is required in comparison with the conventional Carlson process. That is, in the electrophotographic process, the positive charge injected from the backing member due to the first negative charge is held in the interior in the vicinity of the boundary between the photoconductive layer and the transparent electrically insulating layer, and the part of the charge held in the vicinity of the boundary is released as the charge on the surface of the electrically insulating layer is removed due to the second charge. Therefore, a large corona charging current is required to obtain a constant contrast in comparison with the conventional Carlson process.